1. Field of the Invention
The present invention relates to a melt-textured bulk comprising a neodymium-barium-copper-oxide superconductor (hereinafter referred to as "Nd-Ba-Cu-O superconductor", i.e.,one of RE-Ba-Cu-O superconductors), and a process for producing the same.
The present invention provides a bulk superconductor exhibiting a high critical current density over a wide range of magnetic field. Thus, the bulk superconductor can be used in a variety of industries. For instance, the bulk superconductor can be applied to a magnet by trapping a magnetic field, a transferring system by means of magnetic levitation, a flywheel, and an electric power storage system.
2. Description of Related Art
The group of RE-Ba-Cu-O superconductors ("RE" herein stands for a rare-earth element) have recently come to exhibit a high critical current density (hereinafter simply referred to as "Jc"), and can trap a high magnetic field of more than1 T at a liquid nitrogen temperature. These advantages are effected by establishing the following structure; namely: RE.sub.2 BaCuO.sub.5 phases (or RE211 phases), non-superconductive phases, are dispersed in largely grown REBa.sub.2 Cu.sub.3 O.sub.x phases (or RE123 phases) grain, superconductive phases. Melting processes have been developed in order to establish this structure. There are a few manufacturing processes which can be classified into the melting process. The manufacturing processes commonly own the following characteristics; namely: the starting materials are synthesized to have deviated compositions which enable to leave the RE211 phases in the structure of the resulting materials after a heat treatment; the starting materials are heated to a temperature at which the RE123 phases incongruently melt to RE211 phase and liquid phases including Ba, Cu, and O; and thereafter these phases are cooled gradually from a temperature around which the RE123phases solidify, thereby growing large crystalline grains of RE123 phase.
In accordance with the melting processes, it is possible to produce superconductors capable of trapping a large magnetic field which cannot be obtained by permanent magnets. New applications can be expected from such superconductors, and accordingly many researches and developments have been carried out onto the melting processes. As a result, it has been found out that it is necessary to grow the crystalline grains of RE123 phase largely, and to make the RE211 phases, remaining in the RE123 matrix, finer in order to produce a melt-textured bulk with good characteristic. It is believed that the RE211 phases (i.e., the non-superconductive phases) which are dispersed in the crystalline grains of the RE123 phases (i.e., the superconductive phases) work as magnetic-flux-pinning centers substantially. Accordingly, the larger the number of the particles of RE211 phase is, the stronger the magnetic flux can be pinned. Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 5-279,032 discloses that Pt or Rh can be effectively added to make the RE211 phase finer. The publication suggests a process for producing an oxide superconductor which comprises the steps of adding a trace amount of a platinum powder to a raw mixed powder which includes RE, Ba, Cu or their compounds, and mixing and forming the resulting powder; and carrying out a heat treatment onto the resulting compaction. The heat treatment includes partially-melting, and gradually-cooling the compaction.
The conventional melt-textured bulk superconductors exhibit a high Jc in regions of low external magnetic field. However, they tend to exhibit a decreasing Jc as the external magnetic field increases. Thus, there arises a problem in that the conventional melt-textured bulk superconductors cannot exhibit a satisfactory Jc in regions of high external magnetic field.
In addition, researches and developments on Nd-Ba-Cu-O superconductors have been actively carried out recently. In the Nd-Ba-Cu-O superconductors, the Nd elements partially substituted for the Ba-sites of NdBa.sub.2 Cu.sub.3 O.sub.x (i.e., Nd123). Consequently, only the Nd-Ba-Cu-O superconductors of unsatisfactory characteristic have been produced by a heat treatment in air, or in oxygen atmosphere, heat treatment which has been usually used to produce other RE123 oxide superconductors. For example, the thus produced Nd123 exhibits a Tc (i.e., a superconducting transition temperature) lower than those of the YBa.sub.2 Cu.sub.3 O.sub.x superconductors.
However, when the heat treatment is carried out by suppressing the partial pressure of oxygen, the Nd elements are inhibited from substituting for the Ba-sites. Accordingly, the Nd123 was found to exhibit a Tc higher than that of the other RE123. Moreover, Murakami et al. report in pages 368 through 371, volume 64, #4 of the journal of "OHYO BUTSURI (Applied Physics)" Society that the crystalline grains of single Nd123 phase exhibit a high Jc even in regions of high external magnetic field, because the crystalline grains effect magnetic-flux pinning induced by the magnetic field. When the Nd123 incongruently melts to liquid phase and solid phase, the compositions of the solid phase are identical with that of the RE211 phases. However, the solid phase resulting from the incongruent melting of the Nd 123 has been found to have a crystalline structure which differs from that of the other RE211 phases. In order to clarify the phenomena, the phases resulting from the incongruent melting of the Nd123 are called Nd.sub.4 Ba.sub.2 Cu.sub.2 O.sub.10 phase (hereinafter simply referred to as "Nd422" phase).
On the other hand, in the production of the Nd-Ba-Cu-O superconductors, even when the starting materials are synthesized to have deviated compositions and they are subjected to the aforementioned conventional melting processes for producing the other RE123 superconductors, the Nd422 phases (i.e., the non-superconductive phases) remain in the Nd123 phases (i.e., the superconductive phases), but they were not dispersed finely therein. Thus, the structure is hardly established, structure which can effectively improve the Jc of the RE123-based oxide superconductors in the regions of low magnetic field. Namely, non-superconductive precipitates, such as the RE211 phases, are little dispersed finely in the resulting structure of the Nd123 grain.
Thus, the Nd123 reported so far exhibited a high Jc in the regions of high magnetic field, Jc which is effected by their own magnetic-field-induced pinning centers. However, they have suffered from the problem in that they exhibit a Jc lower than that of the other RE123 in the regions of low magnetic field.